This invention is particularly designed for and specifically applicable to hand held welding tools used for delicate welding jobs in a manufacturing operation. An example of such a tool is one used in welding elements such as a wire conductor to a pin conductor mounted on the inside wall of a cathode ray tube. The tool includes a pair of members (which form a hand grip for the operator) that are pivotally joined near one end with the other end (hereafter free end) having the members biased apart. The free end of the tool controls a pair of electrode fingers. An operator simply positions the wire and pin to be welded together between the electrode fingers (using the tool like a "tweezers") by squeezing the electrode fingers together. When the elements are properly clamped together, they are ready for welding and the operator initiates a welding current through the eletrodes to achieve the desired weld.
It has been found that about 5 or more pounds of squeezing force is necessary to insure a secure weld. Thus the necessary squeezing force should be applied before a switch is closed to send the welding current through the electrodes and the squeezing force must be maintained during the brief period of welding. The hand tool briefly described is capable of producing the desired weld. However, an operator of the tool in an assembly line type of production may be required to complete hundreds of welds in a single day. Fatigue alone will cause the operator to reduce the applied squeezing force below the required 5 or more pounds and thereby create bad welds.
The effect of fatigue in the production of inconsistent welds has been recognized heretofore, and a solution that has been posed is to provide air pressure assistance. Thus an air cylinder has been connected to the hand tool to assist in generating an adequate squeezing force. The typical design of air assisted tools, prior to this invention includes the air cylinder with an air hose running from the tool to a remote solenoid operated air valve attached to a source of air pressure. An electric switch is incorporated in the tool handle to trigger the air solenoid valve. Another electrical switch is foot operated to initiate the flow of welding current.
A typical operation for such an air assisted welding tool involves trapping the two parts to be welded between the electrodes by manually squeezing the handles of the tool, then energizing the air solenoid valve by closing the switch on the tool handle, e.g. by squeezing hard on the handle, and finally initiating the electrical current flow by stepping upon a foot switch. As the air solenoid valve is energized, the air flows through the hose and into the cylinder causing the electrodes to exert the proper clamping force. Only then should the foot switch initiate current flow. Since the air hose has enough length and is of a sufficiently small diameter (in order to be adequately flexible) a considerable time elaspes between the closing of the switch on the tool handle and the development of the proper clamping force. Operators will frequently step on the foot switch before the required time elapse and thus before the air hose is completely pressurized. An electrical current flow thus initiated prior to adequate clamping force causes defective welds.
Another design for an air assisted weld tool provides two air operated electrodes that are either opened or closed by air pressure. It is not possible to trap the two parts between the electrodes by manual squeezing of the handles of this design. An electrical switch is closed and full clamping force is applied by the electrodes as soon as the air travels through the air hose. As the air is going through the air hose, an electrical timer runs which causes the weld current to flow as it "times out". This length of time is typically great enough so that the operator will, on occasion, inadvertently release the solenoid air valve before welding has been completed. This reduction of the clamping force as the electrical energy heats the parts will again create a defective weld.